The present invention relates generally to implantable medical devices and more particularly to implantable medical devices intended for use in monitoring a patient's heart rhythm.
Implantable pacemakers and cardioverters monitor the heart's rhythm in order to detect arrhythmias and deliver appropriate therapies to terminate detected arrhythmias. In conjunction with this function, the ability of the device is to store information with regard to monitored heart rhythms has dramatically increased over the past two years. Examples of implantable pacemakers and defibrillators and associated devices which have the capability of storing information related to monitor heart rhythms include U.S. Pat. No. 4,223,678, issued to Langer et al., U.S. Pat. No. 4,295,474 issued to Fischell, U.S. Pat. No. 4,625,730 issued to Fountain et al., U.S. Pat. No. 5,732,708 issued to Nau et al., U.S. Pat. No. 5,669,391 issued to Williams, U.S. Pat. No. 5,522,850 issued to Yomtov et al., U.S. Pat. No. 5,312,446 issued to Holschbach et al. and U.S. Pat. No. 5,776,168 issued to Gunderson, all incorporated herein by reference in their entireties. In addition, there have recently been developed subcutaneously implantable monitoring devices, which do not deliver any anti-arrhythmia therapies to the heart but simply store information regarding a patient's heart rhythms for later uplink to an external device. Such devices are disclosed in U.S. Pat. No. 5,331,966 issued to Bennett et al., U.S. Pat. No. 5,135,004 issued to Adams and U.S. Pat. No. 5,113,869 issued to Nappholz et al., all also incorporated herein by reference in their entireties.
In conjunction with implantable monitoring devices of the types discussed above, storage of information regarding episodes of arrhythmias is typically accomplished in response to either detection of an arrhythmia meeting pre-defined criteria, as described in the above-cited Langer, Yomtov and Nappholz patents, among others, or in response to a trigger signal received from the patient, typically provided by means of an external controller or activator, as disclosed in the above cited Bennett et al. patent. The above cited Fountain et al patent discloses a device having the capability of storing information related to episodes of arrhythmias in response to either a patient trigger or detection of an arrhythmia episode by the implanted device. U.S. Pat. No. 5,931,857 issued to Prieve et al. in particular discloses an implantable defibrillator, which confirms a patient's diagnosis of a need for therapy and in response, delivers the therapy. In the commercially marketed version of the device described in this patent, either confirmation of the patient's diagnosis of arrhythmia or detection of arrhythmia by the device may trigger storage of an associated electrogram.
While devices of the types discussed above are extremely valuable in monitoring the condition of a patient, automatic triggers for electrogram storage are subject to noise, particularly in the context of subcutaneous monitors, and as a result may record electrograms in response to events that are not actually arrhythmias. The use of manual triggers to trigger arrhythmia storage can result in inappropriate storage of electrogram data due to misperception by the patient, or alternatively can result in late initiation of electrogram storage due to a delay between onset of the arrhythmia and the point at which the patient perceives the symptoms of the arrhythmia. In this case in particular, late triggering may have result in the device failing to record the events that are associated with the onset of the arrhythmia, which are often of the most interest.